This application is a 371 of PCT/IL99/00150 Mar. 17, 1999.
The present invention relates to ceramic foams and, more specifically, to a method of producing ceramic foams by liberating volatile reaction products during an inorganic gelation process.
Organic foams are commonly made by introducing a blowing agent (e.g., a supercritical fluid such as CO2 or freon) into a polymer. The polymer is subjected to a rapid pressure drop which causes the blowing agent to form bubbles in the polymer. This process creates a solid containing gas bubblesxe2x80x94namely a foam.
Ceramic foams may be constructed from a variety of materials and may be used in various applications such as thermal insulation, separation processes, catalysis and low dielectric constant materials. In simplified terms, a ceramic foam is a foam where the solid phase is composed of a ceramic material.
The most common method of producing a ceramic foam involves the impregnation of an organic polymer foam (e.g., polyurethane) with a ceramic slurry. The coated organic polymer is dried, then the organic phase burned off. After a sintering step, the resulting ceramic foam is a replica of the original organic precursor.
Another method of producing a ceramic foam, named high internal phase emulsion (xe2x80x9cHIPExe2x80x9d) involves the preparation of a concentrated emulsion containing a continues phase of a polymerizing monomer (e.g., sodium silicate) that is dispersed in a pore-forming phase (e.g., petroleum spirit) with the aid of a surfactant. The continues phase is stabilized by polymerization, washed, and then dried to obtain the foam.
Both of the methods described above produce open-cell ceramic foams. However, these methods do not use the combination of a gas and a liquid phase that is used in blowing agent foam production methods.
Cellular silica and SiC whisker-reinforced cellular silica have been produced using physical blowing agents incorporated into a ceramic suspension. (see Fujiu et al., J. Am. Ceram. Soc., vol. 73, pp 85-90 (1990) and Wu et al., J. Am. Ceram. Soc., vol. 73, pp. 3497-3499 (1990), respectively). This process uses a stabilized aqueous suspension of colloidal silica. The blowing agent is dispersed as small liquid droplets in the suspension with the aid of a surfactant and methanol. The pH of the suspension is adjusted to cause gelation, which is accompanied with a rapid viscosity increase. At this stage, the temperature is raised above the boiling point of the blowing agent thereby producing bubbles in the gel and giving rise to the foam. The duration of the viscosity increase and the setting temperature must be carefully monitored at this stage in order to prevent foam collapse.
In another ceramic foam process (P. Sepulveda, Am. Cer. Soc. Bull., 76, 61-65 (1997)), the foam structure is stabilized by the polymerization of organic monomers incorporated into to the ceramic powder suspension. Initiator and catalyst are added to the system after the foaming stage to induce the polymerization of the organic monomer and the setting of the porous structure.
The above methods have several drawbacks. Most of these methods involve a series of steps (e.g., forming the starting compound, adding blowing agents, etc.). This complicates and increases the cost of the foam manufacturing process. Furthermore, the foams produced thus far often have a 70-90% porosity. Accordingly, a need exists for an improved method of producing ceramic foams, with the option of increasing the pore fraction.
The invention provides a new process for foaming ceramic foams. Significantly, the ceramic foams are produced from a precursor that has an internal blowing mechanism which is activated during gelation. The precursor or mixture of precursors contains at least one ceramic-forming element and liberates at least one volatile reaction product during an inorganic gelation process.
In one embodiment, foaming is based on a precursor containing crystals of the AlCl3(Pri2O) complex. The decomposition of the initial precursor produces polymerizing species dissolved in liquid isopropyl chloride. As long as the solvent and growing AlOxCly(OPri)z species are mixed homogeneously, the boiling point of the solution is raised above the boiling point of the pure isopropyl chloride (35.4xc2x0 C. at 1 atm). Polymerization takes place in the liquid until a critical polymer size is attained, whereupon a phase separation into a polymer rich and solvent rich regions occurs. Since the expelled solvent is suddenly above its boiling point, bubbles start forming instantly. Foam stabilization takes place as a result of gelation in the polymer rich regions which comprise the cell walls in the foam. The net result of the process is then a gelled ultra light foam.
Acceleration of the process is achieved by a mild heating of the precursor (T less than 100xc2x0 C.), but foaming can also take place at room temperature. The heating also affects the cell size, with larger cells produced at lower temperatures. The simplicity of the process is due to the precursor which contains all the necessary foaming functions.
In another embodiment, the mixture of precursors consists of aluminum sec-butoxide and silicon tetrachloride in solution. Upon heating, the solution releases a volatile component (sec-butylchloride) while condensation of AlOx Siy Clz (OR)n species takes place (xe2x80x94OR is the sec-butoxide oxygen donor). As before, the volatile component serves as the blowing agent that creates a foam during the gelation process
The internal blowing mechanism activated during inorganic gelation reactions constitutes the major difference between these processes and conventional processes used to make cellular ceramics. In addition, this fact also explains the inherent simplicity of the process, which can start with a single precursor. The foaming, gelling and drying stages take place simultaneously.
In contrast to other processes (e.g., HIPE, foaming of ceramic slurries), no mechanical stirring is required at the liquid phase due to the homogeneous nucleation of the chemical blowing agent in the method of the invention. This enables the convenient production of thick or thin foamed films from the liquid phase and allows simple in situ preparation of monolithic foams in complicated shapes by a one step procedure. Moreover, the method is simpler than the other methods since it may be self-regulating. Furthermore, the method may produce foams that have significantly higher porosity than conventional cellular ceramics.
In another embodiment, the precursor is placed in a pressure vessel and then the precursor is heated and pressurized to accelerate the transformation of the complex to a solution of isopropyl chloride and partially condensed Alxe2x80x94Oxe2x80x94Al species. In addition, foaming is carried out under pressure; and then the pressure vessel is depressurized. The depressurizing causes either the breaking down of the walls of the foam and/or the expansion the walls of the foam and the creation of new cells. The foam is stabilized by gelation. Furthermore, the step of inducing foaming occurs by depressurizing.
Finally, a method for manufacture of a thermal or acoustic insulator is provided. The method includes the steps of foaming a precursor includes a AlCl3(Pri2O) complex or crystals thereof, heating the precursor, dissolution of the precursor decomposition products in isoproyl chloride, cooling the solution to control the condensation reaction thereby to delay foaming; and spraying the solution on to a surface or into an open atmosphere.
The step of pressurizing the solution can occur prior to the step of spraying.
The step of spraying Includes the spraying onto a hot surface, a cold surface or in a hot atmosphere.